A liquid crystal light control device having a function of controlling the visibility is already commercially available as “switchable light control glass” (“UMU”, registered trademark, manufactured by Nippon Sheet Glass Co., Ltd.). Such glass changes its state instantaneously and reversibly between a transparent state and an opaque state in response to the application of a voltage. In the latter state, light incident on glass scatters to block human visibility in the same manner as in frosted glass, the surface of which is roughened, for example, by sandblasting. Light control liquid crystal devices are employed as partitions for conference rooms, shop windows, vehicle windows and the like.
The liquid crystal light control device has a structure in which a liquid crystal-resin composite produced by dispersion of liquid crystal microparticles in a resin binder (resin matrix) is disposed between two conductive film-coated substrates. In the liquid crystal light control device, the arrangement of the liquid crystal molecules constituting each liquid crystal microparticle changes in response to the application of a voltage to the conductive films. As the arrangement of the liquid crystal molecules changes, the degree of the scattering of light incident on the device also changes corresponding to it. The liquid crystal molecules cause a high degree of scattering of the incident light with no voltage applied (in a voltage free state). Meanwhile, with a voltage applied, the liquid crystal molecules are arranged along the direction of the electric field so that the degree of the scattering incident light is reduced, thereby leading to a transparent state (clear state).
Polyvinyl alcohol, polyethylene glycol, polyurethane, and the like have been studied as the resin matrix for the liquid crystal-resin composite. Among these resins, polyurethane is excellent in view of processability, heat resistance and the like. Although it is possible to use a phase separation process or emulsion process for the dispersion of the liquid crystal microparticles in the resin matrix, the emulsion process has been put to practical use. In the emulsion process, an emulsion including a liquid crystal material is stirred at high speed, thereby allowing the liquid crystal microparticles to be dispersed, that is, the liquid crystals to be “encapsulated”.
It should be noted that the term “encapsulated” is merely a conventional expression that has been used because liquid crystals are observed as if they were enclosed in capsules. Strictly speaking, both in conventional products and the below-mentioned liquid crystal light control device of the embodiments of the present invention, the liquid crystal material is not enclosed in capsules, but it itself is formed into microparticles that are dispersed in a resin matrix. Although the terms “encapsulated” and “liquid crystal capsules” may be used hereinafter according to conventional usage also in this description, they are not intended to mean that capsules are assumed to be present independently of the resin matrix and liquid crystal material.
JP 60(1985)-252687 A discloses a liquid crystal light control device employing polyurethane as a resin matrix. This publication discloses that polymers such as polyethylene, polypropylene, and polyacrylic, and in addition, copolymers such as methacrylate/acrylonitrile, urethane/acrylic, and acrylate/acrylonitrile may be used as a resin matrix. JP 60(1985)-252687 A describes also that a crosslinked structure may be introduced into the resin matrix using a crosslinking agent.
JP 11(1999)-500757 T discloses a method for producing a liquid crystal-resin composite using an emulsion including at least two types of monofunctional (meth)acrylic esters, at least one type of a polyfunctional (meth)acrylic compound, and a photoinitiator, at a predetermined ratio. The invention described in this publication is directed to an improvement of a liquid crystal light control device achieved by focusing on a parameter called an operating field. According to JP 11(1999)-500757 T, the monofunctional (meth)acrylic esters each desirably have a functional group having a chain length of 6 rather than an alkyl group having a chain length of less than 6 in order to keep the operating field low (see FIG. 6 in the publication).
In addition to the above publications, JP 58(1983)-501631 T, JP 5(1993)-66391 A, JP 5(1993)-289068 A, and JP 2004-302194 A also disclose conventional liquid crystal light control devices.
The transparency with a voltage applied is a frequent issue among the properties of the liquid crystal light control devices in the above-mentioned uses. Liquid crystal light control devices appear slightly cloudy even with a voltage applied compared to ordinary transparent glass sheets. The transparency of a liquid crystal light control device with a voltage applied can be improved by making the liquid crystal-resin composite thinner. However, when the thickness of the liquid crystal-resin composite is reduced, the liquid crystal light control device lacks the shielding property with no voltage applied. Thus, there is a trade-off relationship between the transparency with a voltage applied and the shielding property with no voltage applied. Therefore, it is impossible to solve the above-mentioned problem only by adjusting the thickness of the liquid crystal-resin composite.